The production of diglycolic acid by oxidation of diethylene glycol with concentrated aqueous nitric acid is well known. Wurtz, Ann. Chimie [3], 69, 334, 342 (1863).
In a process for the production of ethylenebis (glycolic acid) disclosed in British Patent 639,491, triethylene glycol is gradually added to a 6-molar excess of concentrated, approximately 60% nitric acid at a temperature of 40.degree.-90.degree. C. Nitric acid concentration is maintained constant during the course of the reaction by addition of concentrated nitric acid.
The reaction of diethylene glycol with nitric acid takes place analogously. The process is carried out at the same temperatures; a nitric acid concentration of 35-65%, preferably 50-60%, is employed and a 0.4 - 16-molar excess of nitric acid is utilized. To improve utilization of the nitric acid, oxygen can be added to the reaction mixture to oxidize the thus-formed nitrogen oxides; any nitrogen oxides escaping from the reaction mixture are recycled into the latter. See, DOS's (German Unexamined Laid-Open Applications) 2,206,862 and 2,244,243.
After termination of the reaction, an aqueous reaction mixture, containing approximately 5-30% by weight of HNO.sub.3 and about 20-40% by weight of diglycolic acid, in addition to minor proportions of impurities, such as oxalic acid and glycolic acid, is obtained. According to the state of the art this solution is worked up by evaporating all of the water and nitric acid under vacuum. The thus-separated diglycolic acid is recrystallized once more, if necessary.
However, industrial exploitation of such a process step is impractical, because the physical state of the reaction product changes from liquid phase to solid phase during the evaporation. Due to instability of the thus-separated molten diglycolic acid, it is impossible to discharge the acid as a liquid phase by heating the acid above its melting point. Furthermore, complete evaporation of water and nitric acid from the reaction product requires a high expenditure of energy.
Thus, there is a continuing need for a process in which complete evaporation of water and nitric acid, in the reaction product and the high energy consumption associated therewith can be avoided.